If you want the control channel configuration is combined mode, the SDCCH number will be 12 or 20. The 16 SDCCH channels is the non-combined mode.
If the cell with 4 TRX (32 timeslots) and 16 SDCCH, the available TCH will be 29. All of these TCHs and SDCCHs will occupy 29 + (16/8) = 31 timeslots. The first timeslot of the first TRX in the cell will be used for (1 BCCH + 9 CCCH).